1. Field of the Invention
The present invention relates generally to a system and method for performing cell acquisition in a communication system, and in particular, to a method and system for acquiring cells in a communication system using a frequency overlay scheme (“frequency overlay communication system”).
2. Description of the Related Art
With the development of communication systems, the types of necessary services provided in communication systems are diversified, raising the need for a broadband communication system supporting broadband service. However, due to limited frequency resources in communication systems, broadband communication systems also have limitations on available frequency bands. In addition, because backward compatibility with the previously deployed communication systems should also be taken into consideration, design of broadband communication systems is difficult.
Current broadband communication systems were designed on the assumption that they use different frequency bands. However, development of communication technologies inevitably requires an increase in frequency bands for broadband service, increasing license costs for frequency bands. The increase in license costs causes delay in application of various schemes proposed to provide the broadband service.
As a result, there is a need for schemes for efficiently providing broadband service while overcoming the limitation on the frequency bands, i.e., while solving high-license costs problems for frequency bands. A broadband communication system that is overlaid with an existing communication system in a particular frequency band is a typical scheme. In this case, a mobile station (MS) existing in the broadband communication system is overlaid in the particular frequency band, and an MS of the existing communication system should be able to perform cell acquisition for recognizing a base station (BS) from which they can receive services.
A description will now be made of a general cell acquisition between a BS and an MS.
The BS and the MS should acquire mutual synchronization for signal transmission/reception, and for the synchronization acquisition, the BS transmits a synchronization signal so that the MS becomes aware of the start of the frame transmitted by the BS. Then the MS receives the synchronization signal transmitted by the BS, detects frame timing of the BS from the synchronization signal, and demodulates the frame. Generally, a particular preamble sequence predefined by the BS and the MS is used as the synchronization signal.
A preamble sequence used in an Orthogonal Frequency Division Multiplexing (OFDM) or Orthogonal Frequency Division Multiple Access (OFDMA) communication system should have a low Peak-to-Average Power Ratio (PAPR), and the preamble sequence is used for acquiring coarse synchronization and fine synchronization.
The transmission signal transmitted by the BS is distorted while passing through a wireless channel, and the MS receives the distorted transmission signal. The MS acquires time/frequency synchronization for the received distorted transmission signal using a predefined preamble sequence, performs channel estimation on the synchronization-acquired signal, and demodulates the channel-estimated signal into frequency-domain symbols through Fast Fourier Transform (FFT). After demodulating the frequency-domain symbols, the MS (or receiver) decodes the demodulated symbols into information data using a decoding scheme corresponding to the coding scheme used in the BS.
As described above, the preamble sequence is used for frame timing synchronization, frequency synchronization, and channel estimation. Each BS uses its own unique frame sequence. Therefore, the MS has information on every preamble sequence, detects the preamble sequence having the maximum correlation value through correlation calculation between a preamble sequence included in a received frame and every preamble sequence, and recognizes the BS corresponding to the detected preamble sequence as a serving BS that is providing the service to the MS.
There are various proposed methods for generating the preamble sequence for cell acquisition in general communication systems. However, in communication systems where an existing communication system and a broadband communication system overlaid with the existing communication system in a particular frequency band coexist, the preamble sequence should be designed such that it can distinguish a BS of the existing communication system from a BS of the broadband communication system. That is, the preamble sequence used in the existing communication system should be designed such that it can be used even in the broadband communication system. In addition, the preamble sequence of the broadband communication system should be designed such that its correlation with the preamble sequence of the existing communication system is minimized.
However, there is no specific known method for generating a preamble sequence designed taking into account a communication system where an existing communication system and a broadband communication system overlaid with the existing communication system in a particular frequency band coexist. In addition, the MS, whose operation mode is set to enable communication with the existing communication system, should be able to perform cell acquisition when it is located in the broadband communication system. The MS, whose operation mode is also set to enable communication with the broadband communication system, should also be able to perform cell acquisition when it is located in the existing communication system.